<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        #aaa{
            border: 1px #000 solid;
        }
    </style>
</head>
<body>
    <canvas width="500px" height="500px" id="aaa"></canvas>
</body>
</html>
<script>
    var aaa = document.getElementById('aaa')
    var ctx = aaa.getContext("2d")
    ctx.beginPath()                     // 开始一条新路径

    // ctx.arc(x, y, 半径, 起始角度，结束角度，是否逆时针)
    ctx.arc(200, 200, 100, Math.PI/180*90, Math.PI/180*180, true)
    ctx.stroke();
    
</script>